1. Field of the Invention
The present invention relates to a radiographic image detection apparatus that can perform so-called long-size radiography, in which a plurality of radiographic images are combined to produce a single long-size radiographic image.
2. Description of the Related Art
Conventionally, in medical fields and the like, long-size radiography (longitudinal radiography) is performed in some cases. In the long-size radiography, a radiographic image detector is moved with respect to a subject (patient) to obtain radiographic images at a plurality of positions, and the obtained radiographic images are combined. Therefore, in the long-size radiography, it is necessary that the radiographic image detector can move for a long distance (range) (for example, 180 cm). However, if an accurate stage that does not shift nor rotate even if the radiographic image detector moves for such a long distance is adopted, the cost of the apparatus becomes very high. Therefore, the mechanism of moving the radiographic image detector is provided by an aluminum extrusion rail that also functions as a support pole, and a bearing in many cases. In such cases, a gap (interval, space or the like) is provided between the rail and the bearing so that the movement mechanism operates smoothly even if the aluminum extrusion rail is warped, bent or the like.
Especially, when long-size radiography is performed to obtain an image of the entire vertebral column (spinal column, backbone), a length or an angle between joints is measured. Therefore, highly accurate synthesis of the image is required, compared with ordinary radiography. Especially, a rotation error (inclination) with respect to an axis perpendicular to a detection plane (surface) of the radiographic image detector causes shifting of image-overlapping areas. Therefore, even higher accuracy is required for the rotation error, compared with other kinds of errors.
Therefore, a method for synthesizing an image based on markers provided in a radiographic image detector has been proposed so that a plurality of radiographic images can be combined even if mechanical shift in position, as described above, occurs (please refer to Japanese Unexamined Patent Publication No. 2001-307085 (Patent Literature 1), for example). In Patent Literature 1, markers that block radiation are provided at four corners of the radiographic image detector, and a radiographic image is obtained in such a manner that an image of the marker is included in the radiographic image. Further, rotation of a subject in a plurality of radiographic images is corrected based on images of the marker in the plurality of radiographic images.
However, in Patent Literature 1, it is necessary to arrange the markers in the vicinity of edges on both sides of the radiographic image detector so that the images of the markers are not substantially included in the radiographic images, in other words, so that the images of the markers are not present in the substantial areas of the radiographic images. Therefore, for example, even if a radiographic image of only the vertebral column of the subject needs to be obtained, the entire region of the subject with respect to the width direction needs to be irradiated with X-rays. Hence, there is a problem that the exposure dose of radiation to the patient with respect to the width direction becomes high.